This invention relates to a communication processing technology for a packet relay apparatus having a plurality of packet relay engines or a system.
A network using packet relay apparatuses such as routers and switches is in many cases constituted by a hierarchical type network such as the one shown in FIG. 1. In comparison with a full mesh type construction in which all the packet relay apparatuses are connected through communication cables, the hierarchical type network has the advantage that a network having a greater scale can be constituted by a smaller number of packet relay apparatuses owing to the reduction of the number of communication cables but is not free from the problem that fault exerts greater influences once such a fault occurs in any of upstream packet relay apparatuses.
Therefore, high availability is required for the packet relay apparatuses arranged in upstream hierarchy of the hierarchical type network. Availability is calculated from the probability of the occurrence of system fault and shortness of the propagation time when the system fault occurs. To improve availability, it is very important to quickly restore the system when the fault occurs in addition to the reduction of the system fault occurrence ratio.
Packet relay apparatuses having all the features of Patent Literature 1, Patent Literature 2 and Non Patent Literature 3 are known as means for improving availability. The packet relay apparatus disclosed in Patent Literature 1 has the feature in that the operation can be continued by making all modules redundant and activating a standby module even when a fault occurs in a certain module. The packet relay apparatus disclosed in Patent Literature 2 has the features in that the modules are not rendered redundant as a single apparatus but the packet relay apparatus attains redundancy by causing a plurality of apparatuses to virtually operate as if they were a single unit and can continuously keep the operation by other packet relay apparatuses that are grouped virtually (this technology will be hereinafter called “inter-unit stack technology”) even when a fault occurs in a packet relay apparatus.
Link aggregation disclosed in Non Patent Literature 1 has the feature in that it can secure redundancy of a line by providing means capable of logically gathering a plurality of cables into one and using the cable by connecting a plurality of communication cables connecting the apparatuses. Incidentally, when used in combination with the packet relay apparatuses disclosed in Patent Literatures 1 and 2, link aggregation of Non Patent Literature can improve availability.
FIG. 1 shows an example of a typical hierarchical type network. Referring to FIG. 1, a packet relay apparatus 101 capable of redundancy inside a single apparatus is arranged in the uppermost core layer among three hierarchical structures. This apparatus has the feature disclosed in Patent Literature 1. A packet relay apparatus 102 that can be topped when a plurality of apparatuses is virtually regarded as one unit is arranged in an intermediate distribution layer and has the feature described in Patent Literature 2.
A packet relay apparatus 103 not having the redundancy function of the apparatus is arranged in the lowermost access layer and does not have the technology of high redundancy described in Patent Literatures 1 (JP-A-2007-208579) and 2 (JP-A-2008-78893). Generally, although the packet relay apparatus 101 such as the one described in Patent Literature 1 has high availability, it is arranged in many cases in a hierarchy having a greater range of influences because the apparatus cost is high. Transmission cables are rendered redundant for communication cables connecting each packet relay apparatus by link aggregation 104 described in Non Patent Literature 1 (“IEEE Standard for Local and metropolitan area networks—Link Aggregation” IEEE Std 802.1AX™-2008).